I'm sorry but...
There's *MUCH* more to population capacity than space and current population. There's a reason Earth's history was a *fairly* slow rate of growth prior to the Industrial Revolution. It has to do with food and health care and all that jazz. Just... the Space Age version of that. And remember, based on what we have experienced on Earth, once you get past a certain level of advancement, the birth rate actually goes DOWN, so it makes sense that would stay in place at least at the beginning of space exploration.
Hey thanks for the explanation why it would take so long for population to grow and that it may actually decline as we progress in technology. However this is a video game and I am a Human with an average lifespan of 80 years; I rather not spend my entire life trying to play one round of Stellaris, but based on your information it would take my entire life span to just start colonizing a planet in this game much less win the game (exaggeration).
Honestly the game slowing down this much loses a lot of what made it so interesting and unique to play. The changes are great and open up many options in the game that were previously underperforming like slavery or stealing pops. Additionally they make previous options much more interesting and well adapted to your playstyle like technocracy or aristocratic elite. The one limiting factor is the speed of the game population growing slowly and secondary and tertiary resources decreasing your economic speed results in a very slow and very non-interactive game. Compounding this lack of interactivity is still the undercooked and imbalanced combat system which is still really boring compared to other games. (Still holding that sign up since the games release "Please Make Fleet Combat Fun and Interesting".)
I mean if I forgot the pause button existed for awhile 2.2 has to be really slow. Also slow game + lack of resources due to middle-men resources (Consumer Goods + Alloy) makes the game really short...
I play with many players whose objective of the game is not to have fun (or definition of fun is inflicting pain on others), but to see how fast they can make the other person quit via pounding them into the ground before year 2208 comes up. Which is made easier by 2.2 because if you don't dedicate your all to alloy production the moment you meet a neighbor is the moment when 40 corvettes from a Fanatic Militarists with Distinguished Admiralty hit you in the face long before you had a chance to react. Suddenly your planet of science/unity/growth and defensive fortress is tissue paper against a chainsaw. And because you have so little resource and grow so slowly you can't react to this. One minute you're scouting THEN BAM you get a notification of alien empire meets yours, claim your capital, declaration of war, enemy fleet approaching, star base destroyed, enemy forces landing on planet in about 5 seconds. Kind of why my friend who likes xenophile and federation forming hates me so much. He has no way of countering this other than doing the same focusing on alloy production not expanding too much and going some degree militarists. IT'S GREAT/HORRIBLE these are the fastest games of Stellaris because it boils down to a knife fight in the fog. One second you're reaching out trying to see whose around anxious and scared, the next you're in a bloody corvette stab match with wars lasting a few minutes at best and one side get's all their pops evicted and turned slaves because why deal with slow pop growth when you can just enslave 20 pops from another empire.